1. Field
The subject technology relates generally to optical measurements, and more specifically to systems and methods for calibrating for optical measurement systems that utilize polarization diversity.
2. Background
A waveplate or retarder is an optical device that alters the polarization state of a light wave travelling through it. A waveplate works by shifting the phase between two perpendicular polarization components of the light wave. A typical waveplate is simply a birefringent crystal with a carefully chosen orientation and thickness. The crystal is cut so that the extraordinary axis or “optic axis” is parallel to the surfaces of the plate. Light polarized along this axis travels through the crystal at a different speed than light with the perpendicular polarization, creating a phase difference. Therefore, one of the two perpendicular polarization components experiences a retardation (e.g., slowdown) with respect to the other component in the waveplate. Such polarization diversity is utilized to propagate information in interferometric measurements and sensing systems.
Systems that use polarization diversity to propagate information are sensitive to drift and systematic effects in the birefringence and retardance of the optical components of the system. Calibration is required to characterize and compensate for (e.g., subtract out) these spurious effects. The update rate of calibration is dependent upon the time scales of drift and noise, balanced against sensing requirements. For example, in homodyne metrology where the relative phase of the two polarization components (e.g., in-phase (I) and quadrature (Q) sensing beams) are sensed to determine a position and/or a change in position, the systematic biases in retardance, diattenuation and birefringence need to be calibrated in order to accurately monitor motion to, e.g., a 100 pm level.
In conventional calibration methods, a motion is applied to the entire optical system in order to generate the >1 wave phase shift in the I and Q sensing polarizations. This is usually done by a deliberate actuation of a mirror, which also tends to modulate the data beam as well as the sensing beam. For systems with moderate drift and high precision requirements, these calibration procedures need to be performed frequently, thereby disturbing the normal operation (e.g., measurement or sensing) of the system.
Accordingly, a need exists in an optical measurement system that utilizes polarization diversity to provide a calibration procedure that can be performed without disturbing the measurement of the system.